The fibrinopeptides appear to be involved in hemostasis in the reaction that links the muscle receptor to the contractile mechanism. In the presence of alprenolol, which competes with the binding sites on muscle receptors, the hemostatic effect is reversed causing an increase in bleeding time of scarified rat tail arterioles. These results may be interpreted as receptor modification with a resultant decreased vasoconstriction, if indeed the fibrinopeptides act in the hemostatic mechanism by altering the muscle receptor site and therapy enhancing the biologic activity. The structural modification of fibrinogen by methylation of the carboxyl groups resulted in a polymerized network resembling the fibrin clot. This polymerized fibrinogen possesses similar properties as that of the fibrin polymer such as solubility in 6M urea and 2M acetic acid which is reversible through dialysis. These results indicate that the polymerization mechanism of fibrinogen involves the removal of negative repulsion, generated by carboxyl groups, enabling aggregation to occur. This system may serve as a model for the future characterization of the clinically undesirable physiological thrombus formation within blood vessels.